The invention relates to a device for generating plasma by means of microwaves for CVD coating a substrate, whereby the device comprises a vacuum container into which a reaction gas can be supplied and comprises an electrical conductor arranged in it which conductor is connected to a device for coupling in microwaves.
In order to produce thin layers, coating methods based on the chemical separation of gas phases can also be used in addition to the vacuum metallizing- or sputtering techniques known from the practice. Such coating methods are also designated in the following as CVD methods (from the English: chemical vapor deposition). A solid component is separated off a heated surface of a substrate due to a chemical reaction from the gas phase. At least one gaseous starting compound and two reaction products must participate in the chemical reaction, of which at least one is present in the solid phase. A uniform coating can be achieved with CVD coating methods even in the case of complexly formed surfaces of the substrate.
The temperature load on the substrate can be reduced by a plasma-supported chemical gas-phase separation. To this end a plasma is generated, for example, with microwaves, adjacent to the substrate surface in order to excite the starting compound, customarily a reaction gas, by the plasma and to support the chemical reaction required for the coating.
In a known device for CVD coating (DE 38 30 249 C2) a large-area plasma is generated above a substrate surface to be coated by a number of devices arranged in a matrix shape for coupling in microwaves. The significant construction expense for the coupling-in devices arranged in a matrix shape that are required for being able to generate a planar plasma field that is as homogeneous as possible is viewed as a disadvantage.
A tubular external conductor is known from DE 39 26 023 C2 that is coupled via a slotted opening running axially along the external conductor to a reaction chamber in which a substrate to be coated is located. A plasma can be generated in the tubular external conductor by coupling in microwave energy in the form of microwaves impulses which plasma excites a reaction gas in the vicinity of the slotted opening of the external conductor and supports or makes possible a CVD coating of the substrate.
The previously described devices are not suited or suited only with limitations for generating a coating from an electrically conductive material on the substrate. The chemical reaction supported or started with the plasma has the result that even in the area of the components required for the generation of plasma an electrically conductive coating is separated off that can result after a brief time in a short circuit or at least can adversely affect the generation of the plasma by microwaves. As soon as a spatial separation of the plasma generated by the microwaves from the electrically conductive microwave conductors can no longer be ensured, no spatially propagating microwaves can be formed anymore so that the generation of plasma is interrupted.
Therefore, it is desirable to design a device for the generation of plasma by means of microwaves for being used for a CVD coating method of substrates in such a manner that even a coating of the substrate with electrically conductive material is possible. In addition, a linearly expanded generation of plasma that is as homogeneous as possible should be possible in order that a coating that is as uniform as possible is supported.
In an aspect of the present invention, the electrical conductor is connected on each of its two ends to a device for coupling in microwaves, that the electrical conductor is connected to a voltage source with which a potential difference can be generated between the electrical conductor and the surrounding vacuum container, and that the electrical conductor is insulated against or decoupled from the devices for coupling in microwaves.
An electrical field is generated around the electrical conductor by the potential difference of the electrical conductor opposite the vacuum container and the reaction gas located in it so the electrically charged particles are either moved toward the electrical conductor or rejected away from it. An area surrounding the electrical conductor is created in which less or hardly electrically charged particles reside, as a consequence of which the formation of the plasma is made possible or is supported. It is also conceivable to generate a rapidly changing electrical field by applying a high-frequency alternating voltage so that readily movable particles such as, for example, electrons are moved toward the electrical conductor and a zone poor in electrons is produced while heavy and immovable ions are hardly influenced by the rapidly changing electrical field. A suitable frequency range is the range of radio frequencies between 1 to 200 MHz.
The electrical and/or galvanic insulation of the electrical conductor opposite the coupling in of microwaves ensures that the generation of an electrical field around the electrical conductor does not noticeably adversely influence the generation of microwaves. Instead of an electrical and/or galvanic insulation a possibly commercially available feedthrough filter can also be provided that decouples the microwave infeed from the electrical conductor.
In the following, further embodiments and designs of the concept of the invention are presented by way of example starting from the negative potential of the electrical conductor and can be realized in an analogous manner even in the case of a positive potential of the electrical conductor or given the presence of a high-frequency alternating voltage.
Based on the negative electrical potential of the electrical conductor, electrons from a plasma generated around the electrical conductor are displaced away from the electrical conductor in the radial direction and collect at a distance given by the negative electrical potential around the electrical conductor. The electrons of the plasma that surround the electrical conductor form, together with the electrical conductor, a coaxial, electrically conductive arrangement in which the coupled-in microwaves can propagate. By means of the coupling in of the microwaves at the two ends of the electrical conductor the coupled-in microwave energy can be distributed in a largely homogeneous manner along the electrical conductor and bring about the generation of a correspondingly homogeneous plasma.
In addition, it can be achieved by the negative electrical potential of the electrical conductor that negatively charged particles or electrons are rejected and positively charged ions are accelerated toward the electrical conductor. A suitable presentation of the negative electrical potential can reduce, by the bombardment of the electrical conductor with positively charged ions generated as a result thereof, an adhesion and deposition of the ions striking the electrical conductor with a high kinetic energy and/or already deposited ions can be loosened again by the continuing ion bombardment from the electrical conductor. By presenting suitable environmental conditions and operating parameters, this can achieve a self-cleaning of the electrical conductor that prevents or at least delays a continuously increasing coating of the electrical conductor, in particular in the area of the electrically insulated housing feedthroughs and in the devices arranged in this area for coupling in microwaves. The device can then be used for long periods of time for the CVD coating of a substrate with an electrically conductive coating material without regular interruptions of the coating process being required for cleaning the components necessary for the propagation of microwaves and the generation of plasma. As a rule, a high coating rate is striven for so that only a delayed coating of the electrical conductor is brought about and its coating cannot be completely prevented.
In order to support the self-cleaning of the electrical conductor, the use of a pulsed microwave excitation can be provided that is at the same time also advantageous for many coating processes, in particular for PECVD coating processes.
Since the ion bombardment of the electrical conductor generated by the negative electrical potential causes it to be heated up, is provided that the electrical conductor is a hollow conductor that is connected to a cooling fluid reservoir. The heat generated in the electrical conductor can be reliably removed by a continuous flow through the hollow electrical conductor by a suitable cooling medium such as, for example, air or water.
An embodiment of the concept of the invention provides that the electrical conductor is shaped like a rod. A rod-shaped electrical conductor, in particular an electrical conductor shaped like a hollow cylinder, can be economically manufactured and makes possible a very homogeneous formation of the plasma along the rod-shaped electrical conductor on account of the simple geometric conditions. A generation of plasma that is expanded in a planar manner can be produced by several rod-shaped conductors arranged in parallel and at a distance from each other.
Another embodiment of the concept of the invention provides that the electrical conductor has a curved course. The curved electrical conductor can be arranged inside a substantially level surface and have, for example, a helical or meandering course. In this manner a plasma can be produced that is expanded in a planar manner and is largely homogeneous within a work area with a single electrical conductor already so that a correspondingly homogeneous coating of a substrate adapted in its dimensions to the work range can be achieved.
It is also conceivable to generate complex spatial shapes by a suitable shaping of the one curved electrical conductor or of several curved electrical conductors arranged at a distance from each other so that a largely homogeneous plasma can be generated over or along a complexly curved surface. In this manner even complexly formed substrates can be provided with a coating that is as homogeneous as possible and, for example, workpieces with concave or convex areas are provided with a uniform coating of a conductive material.
In order to prevent that the microwaves coupled in on the two ends of the electrical conductor adversely affect or disturb the negative electrical potential of the electrical conductor relative to the vacuum container, it is provided that the electrical conductor is connected via a feedthrough filter to the voltage source. The feedthrough filter can be arranged either in the area of the device for coupling in the microwaves or, however, in the area of the voltage source or in the course of the electrically conductive connection between the voltage source and the vacuum container.
It is preferably provided that the device for coupling in microwaves widens out like a funnel toward the electrical conductor. In addition, the device for coupling in microwaves can be partially or completely filled with a dielectric material in order to reduce the electrical field at the coupling in of the microwaves and to delay or check a coating with electrically conductive material in this area.
The device for coupling in microwaves can have an outer contour in the shape of a truncated cone or preferably of a horn and with a curved, radially symmetric outer surface.
It is preferably provided that the device for coupling in microwaves has slotted or groove-shaped recesses. The slotted or groove-shaped recesses or notches can be arranged in the radial direction or at an angle to it along closed circumferential lines. A continuous coating with electrically conductive material and the formation of closed, electrically conductive paths over the surface of the device for coupling in microwaves is at least delayed by the surface that is distinctly enlarged in this manner.
It is also conceivable and, with regard to the electrical insulation of the electrical conductor relative to the vacuum housing and the coupling in of microwaves, advantageous that the slotted or groove-shaped recesses are arranged surrounding the electrical conductor in the axial direction. In particular in the case of a concentric arrangement of slotted recesses extending along the electrical conductor and surrounding it a substantially total shading of the slotted recesses takes place in a transitional area to the electrical conductor so that for this reason no continuous coating takes place in this area or only a greatly reduced coating takes place and an electrically conductive connection of the electrical conductor to the housing wall can be significantly delayed or completely prevented in the framework of customary maintenance intervals.
According to an especially advantageous embodiment of the concept of the invention it is provided that the device for coupling in microwave widens out substantially inside the vacuum container. In the area of the housing wall of the vacuum container the device for coupling in microwaves has a comparatively small diameter or a small cross-sectional area so that commercially obtainable sealing devices or sealing components can be used for the pressure density and the electrically insulated fastening of the electrical conductor and of the surrounding device for coupling in microwaves.